1. Field
Example embodiments relate to a semiconductor device, and for example, to a semiconductor light emitting device in which the effect of polarization charges may be reduced.
2. Description of Related Art
Colors of light emitted from a light emitting diode (LED) and a laser diode (LD) vary according to a type of semiconductor compound used. A light emitting device in which a nitride-based semiconductor compound is used may emit blue or violet light.
A conventional light emitting device, for example an LED or an LD, includes an active layer for changing current into light. The active layer has a structure in which at least one quantum well layer and at least one barrier layer are stacked, and the quantum well layer has a single quantum well (SQW) structure or a multi-quantum well (MQW) structure. Because a MQW structure more effectively causes emission at a smaller current, the luminous efficiency of a light emitting device having a MQW structure is higher than that of a light emitting device having a SQW structure.
FIG. 1 is an example diagram showing an energy band of an active layer having a MQW structure of a conventional light emitting device. FIG. 1 shows a state where carriers are not injected into the active layer.
Referring to FIG. 1, the energy band of the active layer includes four barrier layer energy bands BE1, BE2, BE3, and BE4 and three quantum well layer energy bands QWE1, QWE2, and QWE3, which are interposed between the barrier layer energy bands BE1, BE2, BE3, and BE4. The quantum well layer energy bands QWE1, QWE2, and QWE3 are separated from one another by the barrier layer energy bands BE1, BE2, BE3, and BE4. Although not shown, the left side of the drawing indicates a direction in which an n-type contact layer exists, and the right side of the drawing indicates a direction in which a p-type contact layer exists. Reference marks Ec and Ev denote the lowest energy level of a conduction band and the highest energy level of a valence band, respectively.
Before voltages are applied to the n-type contact layer and the p-type contact layer, for example, before carriers (e.g., electrons and holes) are injected into the active layer, the quantum well layer energy bands QWE1, QWE2, and QWE3 and the barrier layer energy bands BE1, BE2, BE3, and BE4 are distorted, as illustrated in FIG. 1. The distortion is based on specific characteristics of a nitride-based semiconductor compound and occurs because charges are generated at an interface between a quantum well layer and a barrier layer. A built-in electric field is generated by the interface charges. Due to the built-in electric field, the transition energy of electrons at the quantum well layer is reduced and a light-emitting wavelength is increased.
The energy band distortion phenomenon caused by the interface charges and the transition energy reduction phenomenon may disappear if carriers are sufficiently supplied to the active layer. For example, if voltages are applied to the n-type contact layer and the p-type contact layer and carriers are sufficiently injected into the active layer, interface charges are annihilated by the carriers and the energy band distortion phenomenon and the transition energy reduction phenomenon may disappear.
However, in a conventional nitride-based semiconductor compound, the mobility of holes is smaller, and the amount of holes that reach the quantum well layers that are closer to the n-type contact layer is smaller. Accordingly, even if voltages are applied to the n-type contact layer and the p-type contact layer and carriers are sufficiently injected into the active layer, the energy band distortion phenomenon of the active layer may occur.
FIG. 2 shows the above result where the energy band distortion phenomenon of the active layer occurs. Referring to FIG. 2, even after sufficient carriers are supplied to the active layer having the energy band of FIG. 1, the energy band distortion phenomenon occurs more pronouncedly in the quantum well layers closer to the n-type contact layer. Therefore, the transition energy reduction phenomenon occurs more pronouncedly and a light-emitting wavelength is increased (e.g., λ1>λ2>λ3) in the quantum well layers closer to the n-type contact layer. The quantum well layers closer to the n-type contact layer may absorb part of the light generated in the quantum well layers closer to the p-type contact layer. Accordingly, the luminous efficiency of the conventional light emitting device including the active layer having the multi-quantum well structure is lowered.
To solve this problem, a method of changing a growth surface of the quantum well layer and a method of doping a material that annihilates interface charges at the interface between the quantum well layer and the barrier layer has been suggested, however, these methods may deteriorate the characteristics of the active layer.